Add COUNTER type with auto-increment and IsNegative field

COUNTER type:
- Only accepts integer values (rejects decimals)
- Auto-increment behavior: each mark adds to existing value
- Supports decrement via negative values (e.g., -2)
- Enforces minimum value of 0 (never negative)
- Default increment is 1 if no value provided

VALUE type:
- Accepts decimal values
- Replaces value (no auto-increment)

IsNegative field:
- New boolean field on Habit entity
- Persisted in database (is_negative column)
- Available in all DTOs and HTTP responses
- Marks habits as "negative" (e.g., candy consumption)

Examples:
- COUNTER: 5 + (-2) = 3, 2 + (-3) = 0, 0 + (-1) = 0
- VALUE: 5000 → 12000 = 12000 (replacement)

internal/application/commands/create_habit.go

@@ -18,6 +18,7 @@ type CreateHabitCommand struct {
 	SpecificDays  []int
 	SpecificDates []int
 	CarryOver     bool
+	IsNegative    bool
 	TargetValue   *float64
 }
 
@@ -46,7 +47,7 @@ func (h *CreateHabitHandler) Handle(ctx context.Context, cmd CreateHabitCommand)
 		return "", errors.ErrInvalidInput
 	}
 
-	habit := entities.NewHabit(cmd.UserID, cmd.Name, cmd.Type, cmd.Frequency, cmd.CarryOver)
+	habit := entities.NewHabit(cmd.UserID, cmd.Name, cmd.Type, cmd.Frequency, cmd.CarryOver, cmd.IsNegative)
 	habit.Description = cmd.Description
 	habit.SpecificDays = cmd.SpecificDays
 	habit.SpecificDates = cmd.SpecificDates